Fix word-spacing adjustment (#69)

[j0hj0h]

Two improvements:

• Line 267: iteration is now going up to 20px. Before it was only 3px which wasn't enough in some cases. In my tests it took never more then 5px to exceed the maxWidth. So 20px should be fine (also for long words)
• Line 147, 163: For browsers with no support for subpixel font scaling, it seems to be necessary to round the values, because they might get rounded incorrectly. At least my Safari (6.1.6) took 0.9px for 0px